The present invention relates, in general, to joining articles using electromagnetic radiation, and more specifically, to welding polymeric articles using electromagnetic radiation through optical aberration.
Various methods of using laser irradiation for thermoplastic welding are known in the art. For example, lasers that emit radiation in the infrared spectral region have been used for thermoplastic welding. As such, several companies have commercialized thermoplastic laser welding systems.
Laser welding has numerous advantages over alternative welding technologies. These advantages include repeatability, welding without surface contact, flexibility in the welding approach, residue free welding, and precision in the welding operation. Another potential advantage of laser welding is that it is possible to minimize distortion of a fragile medium caused by other welding techniques using vibration, heat, or chemicals, thereby maintaining the finished appearance of the medium.
For example, monochromatic radiation can be used to accurately control high resolution welding of small and complex polymer products. These polymer products absorb electromagnetic energy and heat up like a transducer. Polymers have a high heat capacity, poor thermal conductivity, and good infrared absorption characteristics, and as such, they are easy to heat up when they are irradiated with infrared energy.
FIG. 6 illustrates a conventional laser welding system that is used for joining a first article (substrate 600) and a second article (three-dimensional polymeric layer 602). Substrate 600 and three-dimensional polymeric layer 602 are joined at an interfacial layer 604 between substrate 600 and layer 602.
This system includes a laser 610. Laser 610 utilizes an infinite focal point exposing system. As such, the system has no focal point and the laser beam size is controlled by the set of lenses called beam expanders. Laser 610 emits laser beam 614 through projection lens 612. Laser beam 614 is transmitted towards interfacial layer 604. Problems associated with this type of polymer welding system include poor bonding of polymeric layer 602 and substrate 600, upper surface destruction of the polymeric layer 602, and thermal distortion issues.
Referring again to FIG. 6, laser beam 614 may be scanned across interfacial layer 604 to join substrate 600 and polymeric layer 602. In FIG. 6, the beam is shown at the left hand side of interfacial layer 604. For example, in the joining process, laser beam 614 may be scanned from left to right. Throughout this application, this left to right direction will be considered to be the y-axis of the transmitted light; however, the light may be scanned across any axis.
FIG. 7 shows a typical profile of laser beam 614. Laser beam 614 has a substantially round shape 700. As such, laser beam 614 has a cross dimension that is longer at the center (dimension 704) than at other portions of the beam, such as the edges. This longer center dimension results in a longer exposure time when scanning the laser beam in the x-axis direction 702 across the plane of layer 604. This increased exposure time results in a non-uniform energy distribution, adversely affecting the quality of the weld at the interfacial surface.
Another problem with typical laser welding systems is that the laser light has a Gaussian beam structure. This means that the center of the circle (in a circular shaped light beam as in FIG. 7) has a greater energy density than the edges of the circle. This results in a non-uniform distribution of energy because of the relatively high energy density at the center, and the relatively low energy density at edges of the beam. When this non-uniform energy density is coupled with the non-uniform exposure time described above, the quality of the weld is adversely affected.
Lasers have other shortcomings when used for polymeric welding. Polymeric media often have broad spectral irradiation sensitivity and absorb monochromatic radiation with relatively low quantum efficiency. High laser intensity is required to combat the low quantum efficiency and can cause decomposition or distortion of the top surface media.
Conventional laser welding systems, such as the system illustrated in FIG. 6, typically use two types of polymeric materials. The first material is a laser transparent polymer that is placed on top of a second material that is a laser transducer polymer that absorbs the laser light. The laser light source is projected via optics through the laser transparent polymer layer onto the laser transducer polymer layer. The transducer polymer layer absorbs the radiation and is heated to above the melting point of both polymers, causing melting of the polymeric layer 602 and the substrate 600 to form a bond at the polymer interface. For example, referring to FIG. 6, substrate 600 is a laser transducer polymer that absorbs the laser light, while polymeric layer 602 is a laser transparent polymer.
Conventional laser welding systems typically use a cylindrical laser beam profile as shown in FIG. 6 to irradiate the polymer interface. An issue associated with this conventional system is that the laser delivers greater energy at the surface of the polymeric layer 602 than delivered to the polymer interface 604. In some cases, the transparency of the polymeric layer 602 is insufficient and the laser heat damages the surface of polymeric layer 602. Also, heat absorption increases with an increase in thickness of the polymeric layer 602, limiting the thickness of the polymer that can be used in conventional laser welding. Conventional laser welding is typically limited to polymers with high transparency to delivery sufficient heat to the interface to achieve a high quality bond without burning or distorting the surface of the polymeric layer 602.
The practical implications of insufficient melting of the polymeric interface include poor bonding, poor sealing, and inconsistent welding quality. The laser must be carefully focused and accurately controlled to irradiate the small spot at the interface of the polymers to create a quality weld point. A weak weld spot may be formed if the two polymers are not in direct contact at the interface, causing poor welding strength or consistency.
As such, typical through-transmission polymer welding technologies use an infrared laser as the energy source, high infrared transmission polymeric media such as amorphous resin for the top surface (e.g., polymeric layer 602), and transducer polymeric media containing carbon black or infrared absorbers for the bottom surface (e.g., substrate 600).
In summary, known polymeric welding technologies suffer from several deficiencies. For example, the laser beam (e.g., beam 614) has substantially the same energy density at the top of polymeric layer 602 as it does when it reaches interface 604. Actually, the energy density is often higher at the top of polymeric layer 602 than at interface layer 604 because polymeric layer 602 absorbs some energy. As such, surface burning and carbonation of the top surface of the top surface of polymeric layer 602 results. Further, poor bonding often results between polymeric layer 602 and substrate 600. Further still, two distinct materials are typically used for the bonding, for example, laser grade high infrared transmittance amorphous resin for polymeric layer 602, and infrared heat transducer impregnated resin for substrate 600. This results in two distinct high cost materials being used for the bonding.
Further, conventional welding methods often use a transducer coating at interfacial layer 604, resulting in an increased processing time and cost. Further still, because of the lack of irradiation across the interfacial layer 604, the typical molded polymeric surfaces, having poor mechanical precision, can not provide an adequate weld. Furthermore, typical polymers used in the welding process have a very narrow process temperature window, resulting in gases being released, and carbonation occurring.
As such, it would be very desirable to provide a welding system that is simple, cost efficient, and that provides a quality weld at the interfacial surface of two polymeric articles.
To meet this and other needs, and in view of its purposes, an exemplary embodiment of the present invention provides an optical system for joining a first article having a composition and a second article having a composition. The optical system is used to join the first article and the second article at an interfacial surface between the first article and the second article. The optical system includes a radiation source for providing a radiant energy beam for joining the first article and the second article at the interfacial surface. The optical system also includes a first optical device for controlling a depth of focus of the radiant energy beam. The depth of focus corresponds to a depth of a joint between the first article and the second article at the interfacial surface. The first optical device controls the depth of focus as a function of the composition of the first article and the composition of the second article.
In another exemplary embodiment of the present invention, a method of joining a first article having a composition and a second article having a composition is provided. The method relates to joining the first article and the second article at an interfacial surface between the first article and the second article using a radiation source. The method includes a step of providing a radiant energy beam from the radiation source for joining the first article and the second article at the interfacial surface. The method also includes a step of controlling a depth of focus of the radiant energy beam as a function of the composition of the first article and the composition of the second article. The depth of focus corresponds to a depth of a joint between the first article and the second article at the interfacial surface.
It is understood that the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.